The present invention relates to a concentrator for a local area network (LAN) system with loop topology.
FIG. 2 shows a local area network with loop topology using the conventional concentrators as disclosed in, for example, U.S. Pat. No. 4,701,630, "LOCAL AREA NETWORK STATION CONNECTOR" granted to Annunziata et al., issued on Oct. 20, 1987.
In FIG. 2, there are provided three concentrators N.sub.1 -N.sub.3 with the same construction. Workstations, in this example, seven, WS.sub.1 -WS.sub.7 as terminals are connected to each of the concentrators N.sub.1 -N.sub.3. Each of the workstations WS.sub.1 -WS.sub.7 is electrically coupled to a line L via a relay circuit R. To transmit data from one of the workstations, say, WS.sub.2 to another the workstation, say, WS.sub.4, an address code A.sub.4 peculiar to the receiving workstation WS.sub.4 is affixed to the data transmitted from the transmitting workstation WS.sub.2. The transmitted data once received by the adjacent workstation WS.sub.3 the address code A.sub.4 is then checked. When it is determined that the address code A.sub.4 does not agree its own peculiar address code, the transmitted data is transferred to the next workstation WS.sub.4. When the transmitted data is received by the workstation WS.sub.4, the destination address code A.sub.4 is determined to agree to its own peculiar address code A.sub.4, so that the workstation WS.sub.4 finally receives the transmission data to process data according to the input data.
In order to add a new workstation WS.sub.8 to the above-described network, the new workstation WS.sub.8 is connected to the relevant concentrator N.sub.2. A controller (not shown) incorporated in the concentrator N.sub.2 sets the relay circuit R to couple the new workstation WS.sub.8 to the line L. In order to remove one of the workstations, say, WS.sub.2 from the network, the workstation WS.sub.2 is separated from the concentrator N.sub.2. Then, the controller incorporated in the concentrator N.sub.2 resets the relay circuit R so that adjacent workstations WS.sub.1 and WS.sub.3 are directly coupled via the line L.
In the conventional concentrators N.sub.1 -N.sub.3, an AC power source AC is provided for activating the relay circuit R. The AC power source is connected to each of the concentrators N.sub.1 -N.sub.3 via a respective one of cables C.sub.1 -C.sub.3. The AC power source AC enables the switching operations to become rapid and certain, advantageously. However, a cable C.sub.1 -C.sub.3 is required for each of the concentrators N.sub.1 -N.sub.3. In particular, when the size of the network is large and a great number of concentrators are needed, a great number of cables are required. Therefore, the work for the wiring of the great number of cables takes a lot of time.